Currently a gas analyzer is used to measure a concentration of gas of the ppm order contained in air. One of a controlled-potential electrolysis method, a chemiluminescent method, and a level method (Salzman reagent method) is adopted in the gas analyzer. However, in the methods, because the measurement is not performed before the air containing the gas of the examination object is accommodated in a special cell, unfortunately the flowing air containing the gas of the examination object cannot be examined in real time.
A multiple reflection cell method (Wolfgang Demtroder, “Laser Spectroscopy”, Springer Press, P370) or a cavity ring-down method (Wolfgang Demtroder, “Laser Spectroscopy”, Springer Press, P838) can be cited as an example of a method of measuring the gas concentration using the laser beam which has wavelength in the infrared region. A cell is used in measuring the gas concentration, and the cell including reflecting surfaces at both end faces has a long and thin space in which the measuring object is accommodated. The cell is irradiated with the laser beam along a longitudinal direction of the space. The reason the cell having the long and thin space is used is that an optical path of the laser beam in the cell gets lengthened, for example, the optical path is set to 1 to 15 m, enough to accurately examine the trace gas having the low concentration when the measuring object is contained in a low gas concentration of the ppm order in air.
FIG. 7 schematically illustrates a conventional device 50 in which the multiple reflection cell method is adopted. In the device 50, a cell 51 having a long and thin space is used, and mirrors 52 having high reflectances are provided at both ends of the cell 51. The laser beam incident to the cell 51 is reflected many times at both the end faces in which the mirrors 52 are provided, which allows the optical path of the laser beam to be lengthened. A detector 53 detects the laser beam after the laser beam is reflected plural times by the mirrors 52, and a detector 54 detects reference light that is obtained by separating the laser beam immediately before the laser beam is incident to the cell 51. A difference between signal intensity obtained by the detector 53 and signal intensity obtained by the detector 54 is determined to examine the gas. When the laser beam is reflected by the mirror to lengthen the optical path length, an amount of the laser beam absorbed by the gas of the measuring object is increased, so that the absorption amount of the laser beam can be examined even in the trace gas.
FIG. 8 schematically illustrates a conventional device 60 in which the cavity ring-down method is adopted. In the device 60, concave minors 62 and 63 are provided at both end faces of a long and thin space 61, the laser beam is confined in the space 61 to lengthen the optical path of the laser beam. Laser beams p0, p1, . . . , and pN are taken out from the concave mirror 63 by a detector 64 according to the number of reflection times, and a computer 65 computes a gas absorption amount. Thus, the optical path of the laser beam can be lengthened to increase the amount of laser beam absorbed by the gas of the measuring object using the concave mirrors 62 and 63, and the absorption amount of the laser beam can be examined even in the trace gas.
However, in the device 50 illustrated in FIG. 7, the mirror having the high reflectance is hardly prepared, and the width of a light flux of the laser beam is hardly kept constant in the long optical path. In the device 60 illustrated in FIG. 8, the mirror having the high reflectance is hardly prepared similarly to the device 50 illustrated in FIG. 7. In addition, because the concave mirror having the high reflectance is used, unfortunately a laser generating device having high power output is required to introduce the laser beam from the end face into the long and thin space.
Accordingly, the special cell is required to measure the gas concentration. In the device in which the multiple reflection method or the cavity ring-down method is performed using the infrared laser beam, it is necessary to introduce the gas to be measured into the cell in addition to the difficulty of preparing the mirror having the high reflectance and the difficulty of introducing the laser beam. Therefore, a chemical reaction product is hardly measured on site in real time.